The terrestrial snail genus Stemmatopsis is mainly distributed in northern Vietnam and northern Laos. In total five species of Stemmatopsis are discovered, including two described herein. Stemmatopsis arcuatolabris sp. nov. has a small distorted-ovate shell with last whorl protruded quite strongly from ventral side. It has a thickened, expanded peristome with bended palatal side having a bow-like shape. Stemmatopsis dolium sp. nov., has a small, oblique-ovate shell, an approximately quadrangular aperture, and a wave-like shape at the palatal side of peristome.

The shell colour and banding were analyzed in more than 2000 specimens of Cepaea nemoralis collected in 11 localities in Moscow and the Moscow region. In the colonies studied, snails with pink shells usually predominate, most often represented by the phenotype 00000 (complete absence of the spiral bands). In most cases, yellow unbanded shells were absent or represented by single specimens. Brown shells, almost exclusively unbanded, were found only in 5 sites. On the example of colonies from Zagoryanski and Buch (Berlin), as well as Sheremetievski and Dolgoprudnyi, the phenotypic composition of newly formed colonies and colonies that could have been their origins was compared. A decrease in phenotypic diversity of newly formed colonies was noted.

The cellular composition, the morphology of the components, and the screening function and evolutionary changes of the light-insulating apparatus of the camera-like eye of the terrestrial gastropod pulmonate mollusk Arion rufus .were studied and evaluated using light and electron microscopy. According to all the considered characteristics, the light-insulating apparatus of A. rufus is typical for terrestrial pulmonate mollusks. Evolutionary transformations of the light-insulating apparatus, caused by the change in the mollusks' environment, are insignificant and affect the cellular composition, the morphology of pigmented cells and the isolation of the light-sensitive parts of photoreceptor cells from each other.

I propose to restore the old genus name Squamophora for S. oviformis and the described herein S. nierstraszi sp. nov.. A new emendation of the genus Squamophora is provided, taking into account the main features of the shell, girdle and radula that distinguish it from the closely-related genus Loricella. The new species differs from the type species by the sculpture of the dorsal scales and the shape of the radula teeth.

The phenotypic composition of C. hortensis colonies was studied in Western Ukraine (more than 30 thousand adults from 23 settlements and 5 administrative regions) and the Moscow region of Russia (more than 500 adults and juveniles from two colonies in Moscow and Vidnoe). Most Western Ukrainian colonies were characterized by the absence of pink shells, the high frequencies of unbanded shells (70% or more), as well as the white ground colour in all banded shells. It is supposed that the latter feature makes it possible to reliably identify the colonies formed by the descendants of the primary introduction of C. hortensis into Western Ukraine, which occurred in the 20th century, from later and independent repeated introductions of this species. Unlike Western Ukraine, snails with yellow banded and pink shells were found in both colonies from the Moscow region. Pink banded shells were registered only in Moscow as well as in two colonies from Lviv. In two of the three colonies mentioned, part of the pink unbanded (Moscow) or all pink shells (Lviv) had a dark lip.

In the present work we investigated the influence of three days anoxia on hematological parameters, morphological and functional characteristics of eryhroid cells of alien bivalve Anadara kagoshimensis. Oxygen concentration in seawater was decreased by bubbling with nitrogen gas for 5 h. Temperature was maintained at 20±1oC and photoperiod was 12h day: 12h night. Extrapallial fluids were sampled by a puncture of extrapalial cavity. Three-day exposure to anoxia caused pronounced cellular responses. At the organismic level changes were not observed, as hemoglobin concentration the total number of erythroid cells and mean hemoglobin concentration (МСН) remained at the level of normoxia. We observed an increase of cellular anomalies, i.e. shistocytes and erythroid cells with polymorphic nuclei, and cells undergoing reactive amitotic division, which resulted in formation of binuclear cells. Nuclear volume (Vn) increased for more than 40 % compared to control level. This increase depended on the duration of anoxia. Changes in cellular volume (Vc) possessed a complicated manner. At the end of experimental period (3rd day of anoxia) nucleo-cytoplasmic ratio was 36% lower comparing to normoxia. Exposure to anoxia did not cause mortality of erythroid cells. The number or red blood cell shades observed on slides remained at control level.

Sexual dimorphism is the condition where individuals of different sex in the same species exhibit different characteristics beyond the differences in their sexual organs. In this study, individuals of a viviparid species Filopaludina martensi from the Kwai Yai River in Kanchanaburi Province (Thailand) were examined for eight shell and operculum characteristics. Sexual differences were observed in size of shell and operculum, with females being larger than males. The results indicated that morphometric analyses are useful to detect subtle differences between sexes in this species.

Using the methods of light and electron microscopy, the morphological properties were studied, and the evolutionary changes in the vitreous body of the camera-like eyes of some species of terrestrial gastropod pulmonate molluscs were traced. It was demonstrated that all the considered properties of the vitreous body of molluscs are typical for terrestrial pulmonates. In the course of evolution, as a result of the land development by mollusks, the properties of their vitreous body have hardly changed.

Three small but abundant in specimens isolated colonies of Helix lutescens have been found in the eastern part of Moscow City in the central part of European Russia for the first time. This is an easternmost known colony of the species. Differences from the related and similar small form of H. pomatia are given and discussed.

Two colonies of an invasive West European land snail Cepaea nemoralis are reported from Kyiv region of Ukraine, as well as a third locality where snails were present in 2019, but no living individuals were found in 2020. One more locality is reported from Odessa City in Southern Ukraine. These localities are the most south-eastern ones for the range of C. nemoralis and are the first known records of this species in Ukraine outside its western regions, with closest previously published confirmed colonies in more than 400 km. Distribution of this species in Eastern Europe is discussed, it is argued that invasion of C. nemoralis is probably driven by the same processes as invasion of the Spanish slug, Arion vulgaris, that invaded Eastern Europe during the last decades.

The importance of trees for an invasive Mediterranean cellar slug, Limacus flavus, was studied within one block of a residential area in Kyiv city (Central Ukraine) from spring of 2020 to spring of 2021. Slugs tended to occur in the old poplars (Populus nigra var. italica). Live animals or their traces were found on the 71 of 320 poplars and on 17 trees of other species in the studied area. Slugs often go up to at least 12 m above the ground on these poplars and feed on the lichens there. Animals breed and spend the daytime inside at least some of these trees under the bark. Slugs were wintering in some of these trees in 2020-2021 and probably also at least in 2019-2020, while previously L. flavus was reported from Eastern Europe only in connection to cellars, basements and greenhouses. Apparently these slugs are occupying additional habitats in response to climate change and their occurrence in the trees may become common in Eastern Europe with the warming of climate. The colouration of L. flavus and closely related L. maculatus is discussed. Various differences of the colouration were suggested in the literature to distinguish the two species, most notably the central light stripe on the back of L. flavus, but this character is absent in most of the studied specimens and, therefore, the overall colouration overlaps in the studied populations of the two species.

The spat of the scallop Flexopecten glaber were collected in cages with the Pacific oyster Crassostrea gigas cultured on a mussel-and-oyster farm (outer roadstead of Sevastopol Bay). For two years they were reared in plastic cages at a depth of 2–3 m. The cages were periodically withdrawn to measure the size and weight parameters of the molluscs: the length (L, mm), height (H, mm), and width (D, mm) of shells and the total live weight (W, g). The correlations among the parameters under study are presented. The changes in the indices of the frontal (D/L) and sagittal (H/L) curvatures, convexity ((H+D)/L) and conditional volume (H×D×L/1000) of the shells in ontogenesis are shown. Upon reaching a shell length of 30–35 mm, the allometry of the volumetric and weight growth of molluscs changed from positive to negative. The largest values of the shell convexity index were registered in the same length range. A conclusion about the interval-type growth of F. glaber in linear size and weight is made. It is suggested that the optimal strategy of shell formation in F. glaber in ontogenesis implies the ripening and the first reproduction of the molluscs occurring at the highest volumetric characteristics of the shell. The relationships for the linear size and weight growth of F. glaber in the first two years of life are obtained. It is concluded that the scallop F. glaber should be considered as a possible element for the diversification of the existing aquaculture of molluscs (mussels and oysters) off the coast of Crimea.

Species of the genus Mastoniaeforis are mainly distributed in the Indo–West Pacific with ten species being recorded in the world. To date this genus was not recorded in Vietnam. Specimens of three species, M. chaperi, M. lifuana and M. speciosa, were collected in two field trips from May, 2019 to October, 2020 in Truong Sa archipelago (Spratly Islands), Khanh Hoa Province, Vietnam. The diagnostic characters, distribution, and illustrations of three Mastoniaeforis species in Vietnam are provided.

The contribution to malacology made by the eminent Russian scientist and traveler, Alexander Theodor von Middendorff (1815–1894) is reviewed. Middendorff’s research is rightly considered the highest achievement of Russian malacology in the 19th century. It is shown, despite the relatively short timespan of his malacological activity, Middendorff could reach substantial progress in the knowledge of the Russian malafocauna, both marine, and continental, and authored more than 15 malacological publications, including a series of fundamental systematic works. Middendorff’s views on taxonomy, variability, and zoogeography of molluscs are discussed, and the research program in malacology, proposed by him, is reviewed as well as the impact of his studies on the further development of malacology in the Russian Empire. The full list of all molluscan taxa described by Middendorff is provided as an “Appendix” to the article.

The species composition and distribution of freshwater gastropods in the western part of the Kola Peninsula and northern Karelia has been studied. The explored region lies in the far north of Europe and covers several landscape zones: from the northern taiga to the tundra. In sum, seventeen species found, for each of them the distribution maps and photographs of intraspecific forms of conchological variability are presented. The taxonomic remarks are also given when appropriate. Gyraulus acronicus (Planorbidae) and Ampullacena balthica (Lymnaeidae) are the most common species in the studied region. Two species (Ampullacena balthica and Galba truncatula) have been recorded in the Barents Sea islands off the Kola Peninsula. A single non-indigenous species, Physella acuta (Physidae) has been registered in the studied territory. Based on the results of a comparison of the faunas of gastropods from various areas within the whole territory studied and other regions of northern Europe and Western Siberia, it has been shown that the most dramatic reduction in the species composition occurs during the transition from taiga to tundra landscapes.

On the territory of Republic of Armenia a new species of land snails has been found. Illustrated description is presented. Relationship between five close related Transcaucasian species of Armenica is discussed. New species differs from closely related A. unicristata in having more large dark shell with well developed axial striation and character of clausilium.

Based on a study of samples made by the author, 70 species of freshwater bivalves belonging to 6 genera and 2 families are recorded for the Taz River basin (north of Western Siberia). An annotated list of bivalves of the Taz basin is provided, with data on species’ range, their findings in Western Siberia and within the studied area. Some information about bionomics and abundance of bivalves are given. 45 species are for the first time registered in this basin. Most studied molluscan communities are characterized by low species richness; the distribution of species by their occurrence was extremely uneven. 22 species are characterized as rare. The highest species richness of bivalves was found in river channels, rivers and brooks; the lowest – in temporary habitats. The core of the fauna is constituted by species with broadest range (cosmopolitan, trans Holarctic, trans Palearctic) as well as by taxa with European Siberian type of distribution. From the taxonomic point of view, the bivalve fauna of the Taz basin is relatively separated from the faunas of other river basins of Western Siberia and is more similar to the fauna of the Lower Yenisean zoogeographic province (sensu Starobogatov [1986]).

TEM and SEM electron microscopy have been used to study osphradia in 6 species of marine Caenogastropoda. The ultrastructural features of mechanoreceptor cells that perform the Littorina osmoreception function in osphradium organs are presented. Mechanoreception is based on a possible change in the volume of cisterns of microvilli of supporting cells, which can be transmitted by the cilia of nearby mechanoreceptor cells. These cells obviously, have mechanosensory channels on the apical surface. It has been first discovered in predatory molluscs actively searching for food, that single receptor cells with a mobile sensilla consisting of several cilium were joined together. They are located along the groove zone and follow the direction and force of the movement of water along the osphradium lamellae.

A new species Acrotoma (Acrotomina) mallabica sp. nov. from the Western Caucasus is described. The morpho-anatomical relations of the new taxon with closely related species are discussed.

The invasion of the predatory gastropod mollusk Rapana venosa, known as veined rapa whelk, in the Black Sea and its outspread in many regions of the World Ocean have led to complex structural changes in the communities of bivalve mollusks. When capturing a prey, the adult rapa whelk excretes a biotoxin from the hypobranchial gland that causes severe paralysis of the musculature of bivalve mollusks. Under experimental conditions, it was established that the biotoxin, after the short-term exposure of the fertilized eggs of the mussels Mytilus galloprovincialis to it, has a mutagenic effect, causing chromosomal aberrations in the embryos. The acentric groups of chromosomes, single and multiple chromosome bridges, chromosome lags and polyploidy indicate transformation or destruction of threads of the achromatin spindle in the mitosis anaphase. The chromosome fragmentation and emergence of ring chromosomes in the metaphase and anaphase of mitosis of mussel embryos are the consequence of the partial destruction of chromatin. The underdeveloped shell, anomalous development of D-veligers’ velum, and the changed form of the ‘eye spots’ and hepatopancreas of the larvae at the veliconcha stage result from chromosomal mutations during the embryo development. The mechanism of the rapa whelk biotoxin action on the chromosomal apparatus of mussel embryos is discussed.